Xbox One's massive, custom CPU can pick you out of a lineup

One of the problems that Microsoft had to solve when developing the Xbox One game console was a simple one: how to tell apart the player using the interactive gaming console from his friends, the lamp, the couch, and the coffee table. To solve that issue, Microsoft developed its own light sensor that can figure out what it’s seeing by a variety of calculations.

In talking up the innards of its upcoming gaming console, Microsoft didn’t reveal any new capabilities of the One—due this November—during a presentation at the Hot Chips conference Monday at Stanford University. But company executives discussed the so-called “speeds and feeds” of the main Xbox One processor in detail, as well as the problems that the new Kinect team had to overcome.

MicrosoftThe Xbox One system diagram.

One massive chip

Physically, the system-on-a-chip at the heart of the Xbox One is 363 square millimeters. But the real whopper is the amount of logic integrated within it: 5 billion transistors. Although Wikipedia isn’t necessarily the final arbiter, the Xbox One is possibly the largest chip manufactured to date, with a comparable number to Intel’s 64-core Xeon Phi coprocessor for supercomputers, and easily topping Intel’s 8-core “Poulsen” Itanium that the chip maker launched in 2012.

MicrosoftThe Xbox One system-on-chip.

Unfortunately, Microsoft didn’t disclose how much power that chip will consume. However, the chip will be power gated down to 2.5 percent of its power, said John Sell, a hardware architect at Microsoft who gave the Xbox One silicon presentation at Hot Chips. It will be manufactured in 28-nm process technology by TSMC, Microsoft’s foundry partner.

Nor did Microsoft disclose how fast the chip will run. All told, the Xbox One system contains 500GB of storage, 8GB of DDR3 memory and 47MB of on-die storage that can be used as cache memory.

The system CPU is based on the AMD Jaguar processor architecture; the system uses eight cores with 32 KB of instruction and data cache, with four of the cores connected to a shared 2 MB shared level-2 cache.

MicrosoftThe graphics unit of the Xbox One.

The Xbox One chips are highly customized, Sell said. One significant change from the previous-generation Xbox 360 is that the graphics core is integrated with the CPU, as are most of today’s PCs chips. Sel referred to the GPU chip as a “DirectX 11.1+” chip, as it contains optimized graphics processing unique to the Xbox One. “There’s more than a CPU’s worth of processing in there,” Sell said.

Kinect challenges

Each Xbox One will contain a Kinect depth camera, designed by Microsoft. (The camera used in the Xbox 360 was designed by PrimeSense, an Israeli developer.)

Patrick O’ Connor, another system designer for Microsoft, explained that the chip can resolve players less than 1 meter away, and players at more than 2 meters away, with a 70-degree field of view. Microsoft also has to take into account a whole range of environments—from players that like to play games in the dark to broad daylight.

The Kinect on the Xbox One actually “illuminates” the room with its own modulated light. What the camera’s 512 x 424 sensor “sees” is the reflected light, which will be out of phase depending on the way it reflects from objects. The Xbox One then adds or subtracts their values to weed out the ambient light, determine the player’s location and motion, and what is the “active” image. Each Xbox One can distinguish up to six different players, O’Connor said, but exactly “who” is playing at any one time takes some figuring.

MicrosoftSidelighting could confuse the Xbox One camera without supplemental illumination, Microsoft said.

What the Xbox One can do, however, is determine, on a per-pixel basis, how best to “see” the object. Since the camera can’t actually lengthen its exposure or widen its aperture, it needs to use different shutter times, before picking the best one. Combining the Kinect’s own illumination with intelligent “reading” of the image eliminates common problems, such as sidelighting that can confuse the camera, O’Connor said.